The present invention is directed to a circuit for controlling the frequency of a transmitter. In particular, it is directed to a servo loop to control the frequency of a transmitted signal and keep it equal to the frequency of a prior signal. And more particularly the present invention is useful in a circuit in a marine radar beacon. A marine radar transponder is a microwave transmitter which is triggered to a response by a radar pulse or racon from a ship resulting in a reply signal which locates and identifies the racon. The return signal must be synchronous with, and generated at the frequency of the received signal.
The present invention is directed to a servo loop with a self-calibrating output which is preset. The servo control loop has a controller which doubles as an integrator and an amplifier. The incoming radar signal is received and stored digitally and becomes a reference which does not drift. When in the amplifier mode, the offset and gain of the amplifier are digitally controlled. When the racon begins transmitting, the controller is switched to the integrator mode to slew the output to the correct frequency.